There exist methods to cancel aliased echo peaks created by aliasing phenomenon according to Shannon theory in an estimated channel impulse response of a telecommunication channel.
For example, US 2005/0213680 in the name of Atungsiri et al. describes such a method to cancel aliased echo peak in an estimated channel impulse response of a channel used in an OFDM (Orthogonal Frequency Division Multiplexing) telecommunication system.
One can also refer to this document to obtain more information on OFDM receivers.
In wireless communication system, the wireless signal emitted by an emitter may follow different paths before reaching an antenna of a receiver. In this specification, the path through which is received the signal with the highest power is called the main path. The other paths are called secondary paths.
The signal that follows a secondary path is called an echo.
Typically, the main path and the secondary paths do not have the same length.
Thus, the signal is received at time t1 when it follows the main path and at time t2 when it follows the secondary path. Time t2 may be in advance on time t1 and in this case the echo is called a “pre-echo” or behind time t1 and in this case the echo is called a “post-echo”.
In a real channel impulse response (CIR), an echo corresponds to an echo power peak at a time distinct from the time where appears the highest power peak corresponding to the signal that follows the main path. This echo power peak is called an echo peak in this specification whereas the highest power peak corresponding to the main path is called the main peak.
An echo peak is strongly correlated to the main peak because the echo peak is created by a copy of the signal that creates the main peak, but with an offset in time.
As explained in US 2005/0213680, aliased echo peaks can appear in an estimation of the channel impulse response (CIR) when the estimation is built from an insufficient number of sampled data. This happens according to the well-known Shannon's theory. The estimation of a channel impulse response is built over a specified temporal window. In OFDM communication systems, the temporal window is known as IFFT (Inverse Fast Fourier Transform)-window, for example.
Thus, it should be understood that an aliased echo peak corresponds to a real echo peak but is not placed at the right place in the temporal window over which the channel impulse response is estimated. From this aspect, an aliased echo peak is distinct from power peaks known as “ghost peak” or “replica peak” or “image peak” that do not correspond to a real echo. In fact, ghost peak, replica peak and image peak are generated by parasitic effects like Doppler effect but not by echo. This is an important difference because to correct the estimated channel impulse response, the ghost peak and the like have to be cancelled whereas the aliased echo peaks have to be replaced at the right place in the estimated channel impulse response.
However, the aliasing of an echo peak only appears if the time interval T between the echo and the signal that follows the main path is greater than a predetermined limit. In fact, if time interval T is inferior to this predetermined limit, the echo peak appears at the right position in the estimated channel impulse response. In this situation, the echo peak is called real echo peak in this specification.
In contrast, if time interval T is greater than the predetermined limit, aliasing occurs and the estimated channel impulse response includes an aliased echo peak instead of a real echo peak.
Most of the time, the value of time interval T is not known. Thus, it is difficult to discriminate aliased echo peaks from real echo peaks.